The overall goal of this proposal is to develop novel dental composites with significantly reduced contraction stress and improved fracture toughness and wear resistance. The main reasons for clinical failure of dental composites are secondary caries, wear and fracture. The expanded use of composites in posterior teeth is limited by their technique sensitivity which is related to excessive polymerization contraction stress and inadequate resistance to wear and fracture under occlusal stresses in moderate to large cavities. The investigator has discovered that the addition of non-bonded microfillers to microfill or hybrid dental composites reduces contraction stress and increases fracture toughness, without negatively affecting other properties. It is hypothesized that these improvements are due to internal stress relief and the activation of additional toughening mechanisms. The investigator has developed specific hypotheses about the effect of nonbonded microfillers on composite behavior, and proposes to make specific formulations of composites containing bonded (functional silane treated) and nonbonded microfillers to test them. The clinical significance of this work is that it addresses the main problems with current dental composites, and proposes to use simple modifications of currently accepted materials. There are five specific aims. Aim 1 proposes to measure the contraction stress produced during curing of the new composites. The specific hypothesis is that the addition of some nonbonded microfiller particles provides internal sites for stress relief, thus minimizing stress within the curing composite. The influence of filler volume, proportion of bonded and nonbonded particles, resin composition and the C-factor will be studied. Aim 2 proposes to measure the fracture toughness and in vitro wear resistance of the new composites. The specific hypothesis is that additional toughening mechanisms are achieved by the addition of nonbonded microfillers, resulting in composites with enhanced fracture toughness and wear resistance. The influence of filler volume, proportion of bonded and nonbonded particles, and resin composition will be studied. Aim 3 proposes to measure and compare the elastic modulus, transverse strength, degree of conversion, polymerization contraction, density, and opacity of the new composites with commercial materials. The specific hypothesis is that the inclusion of nonbonded microfillers does not adversely affect the physical or mechanical properties of the composites compared with commercial materials. The properties of the new composites will also be tested after long-term aging in water to verify that they do not deteriorate with age. Aim 4 proposes to evaluate in vivo the abrasive wear and marginal integrity of the composites and compare them with commercial materials. The specific hypothesis is that the inclusion of nonbonded microfillers improves the in vivo wear resistance and marginal integrity of the composites compared with commercial composites and controls containing bonded microfillers. Aim 5 proposes to evaluate the in vitro marginal leakage of these composites in Class II restorations with interproximal margins extending below the cemento-enamel junction. The specific hypothesis is that composites with reduced polymerization stress during curing will result in restorations with significantly reduced microleakage as compared with commercial composites and controls containing bonded microfillers.